Make libbinder a shared library only (not static).

Letting the library be statically linked does a couple of bad things:
- (importantly) allows its internal implementation details to become frozen
- (still important) uses extra space on this device

For these reasons, it is being made shared-only.

Test: boot Pixel 2
Bug: 111454508
Change-Id: I787b9979646230485f7a2b3aa8ad949c7649bc88