Make bitmap nullable in ViewCapture

Fixes local compilation in SysUI studio.
The callback interface has @NullableDecl
annotation that Kotlin compiler treats as
Kotlin nullable.
Not sure why it compiles successfully in
soong though (different compiler version?).

Bug: 236188940
Change-Id: Ia8cf1dd25c3ea6a162ad3dddd58b4bf4c0f889a9
Test: build from sysui studio