perf(Android): recycle replaced shadow bitmaps

[OskarEichler]

Performance problem and fix

ShadowLayout regenerates its cached bitmap after layout/appearance changes. The previous bitmap was overwritten without being recycled, so each regeneration left its native pixel allocation for GC/finalization instead of releasing it deterministically. A full-screen-width shadow cache can make that churn substantial during repeated layout changes.

This change centralizes cache teardown:

• detach the bitmap from the reusable Canvas
• recycle and clear the previous cache before replacement
• use the same cleanup when the view detaches
• mark the shadow dirty after detach so a reattached view rebuilds its cache

The temporary alpha bitmap was already recycled and remains unchanged.

Regression coverage

The new Robolectric test renders a shadow, invalidates layout, renders again, and verifies the first bitmap was recycled before the replacement became active. The exact baseline leaves it unrecycled.

Verification

• focused regression: 1/1 passed
• full Android unit suite: 698 passed, 2 skipped, 0 failed
• Android debug Kotlin/Java compilation passed
• git diff --check passed

Breaking changes

None. Rendering output is unchanged; native bitmap memory is released promptly.