Set buffer size to match Linux pipe internal buffer when reading/writing...

Set buffer size to match Linux pipe internal buffer when reading/writing from/to pipes in Backup/Restore flow.

ag/21072118 used of feature flags to control buffer sizes, which are now deprecated. Those flags were also never used.
This change makes use of trunk-stable frozen flags to set the buffer size to match Linux pipe internal buffer when reading/writing from/to pipes in Backup/Restore flow.
See 'Pipe capacity' section in https://man7.org/linux/man-pages/man7/pipe.7.html

Bug: 265976737
Test: Ensure that the change builds (m -j frameworks/base)
	Ensure CTS and GTS B&R tests pass
Change-Id: I8bb96e2e273b0c7eb4809af985fe322e49bb0268