Optimize and clean up Utils.java

- Hide constructor
- Make BD_ADDR_LEN public for reuse
- Avoid duplicate calculation of BigInteger.valueOf
- Remove unused byteArrayToString(), callerIsSystemOrActiveUser()
- Improve encapsulation

Bug: 394958283
Test: atest UtilsTest
Flag: EXEMPT refactor no-op
Change-Id: Ib411106b559afaa930599e5891721c3be4884d51