Make powerof2 macro ubsan safe

Subtracting one from the smallest value expressable by the provided
variable could cause an underflow operation. In particular, this is
problematic when code similar to:

  uint64_t foo = 0;
  if (powerof2(foo)) {
    ...;
  }

is run with integer sanitization enabled. The macro would subtract one
from zero, underflowing and triggering the sanitizer.

Make the powerof2() macro ubsan safe, by explicitly handling underflows.

Note: This change DOES NOT make powerof2() accurate. We continue to
falsely return "true" for 0 and negative numbers (see attached tests).

Found while investigating Bug: 122975762
Test: see added testcase
Test: atest ziparchive-tests

Change-Id: I5408ce5c18868d797bcae8f115ddb7c4c1ced81e